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arXiv:physics/0304031 · Physical Review Letters

Forbidden Transitions in a Magneto-Optical Trap

Mishkatul Bhattacharya, Christopher Haimberger, N. P. Bigelow

Published Nov 20, 2003• 24 citations
arXivHTMLPDF

Abstract

We report the first observation of a nondipole transition in an ultracold atomic vapor. We excite the $3P\mathrm{\text{\ensuremath{-}}}4P$ electric quadrupole ($E2$) transition in $^{23}\mathrm{N}\mathrm{a}$ confined in a magneto-optical trap, and we demonstrate its application to high-resolution spectroscopy by making the first measurement of the hyperfine structure of the $4{P}_{1/2}$ level and extracting the magnetic dipole constant $\mathrm{A}=30.6\ifmmode\pm\else\textpm\fi{}0.1\text{ }\text{ }\mathrm{M}\mathrm{H}\mathrm{z}$. We use cw optical-optical double resonance accompanied by photoionization to probe the transition.

DOI
10.1103/physrevlett.91.213004

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Abstract Results of spectroscopy experiments using the 5 P 3 / 2 → 6 P 3 / 2 electric dipole forbidden transition in cold rubidium atoms are presented. Production of this forbidden transition is detected by observing the emission of the 420 nm fluorescence photons that result from the decay of the 6 P 3 / 2 state into the ground state. The experiments are performed under the steady state operation conditions of a magneto-optical trap (MOT), and thus provide non-perturbing information on the atom-light system. The hyperfine structure of the 6 P 3 / 2 level is completely resolved, and the fluorescence peaks show the expected Autler–Townes (AT) splitting of the emission lines. This hyperfine structure is used to calibrate the frequency scale of all spectra recorded. A combination of this absolute frequency scale and an expression for the AT profile allows an absolute determination of the effective Rabi frequency and detuning of the MOT. The behavior of these AT doublets is studied as a function of frequency detuning and also as a function of the trapping light intensity. The experiments also take full advantage of the strong polarization dependence of the relative intensities of the hyperfine fluorescence components. This study results in a sensitive probe of the relative populations of the magnetic sublevel projections relative to the trapping magnetic field gradient. An almost isotropic population distribution was found, but small deviations from isotropy could be determined with this electric dipole forbidden probe.
This paper presents a general review of the results of the experimental and theoretical work carried out by our research group to study the 5P3/2→6PJ electric quadrupole transition in atomic rubidium. The experiments were carried out with room-temperature atoms in an absorption cell. A steady-state population of atoms in the 5P3/2 excited state is produced by a a narrow-bandwidth preparation laser locked to the D2 transition. A second CW laser is used to produce the forbidden transition with resolution of the 6PJ hyperfine states of both rubidium isotopes. The process is detected by recording the 420(422) nm fluorescence that occurs when the atoms in the 6PJ state decay directly into the 5S ground state. The fluorescence spectra show a strong dependence on the relative polarization directions of the preparation laser and the beam producing the forbidden transition. This dependence is directly related to a strong anisotropy in the populations of the 5P3/2 intermediate magnetic substates, and also to the electric quadrupole selection rules over magnetic quantum numbers. A calculation based on the rate equations that includes velocity and detuning dependent transition rates is adequate to reproduce these results. The forbidden transition is also shown to be an ideal probe to measure the Autler–Townes splitting generated in the preparation of the 5P3/2 state. Examples of spectra obtained with cold atoms in a magneto-optical trap (MOT) are also presented. These spectra show the expected Autler–Townes doublet structure with asymmetric line profiles that result as a consequence of the red-detuning of the trapping laser in the MOT.
Abstract We present the first evidence of excitation of the 5 p 3 / 2 → 6 p 1 / 2 electric dipole-forbidden transition in atomic rubidium. The experiments were done in a rubidium vapor cell using Doppler-free optical–optical double resonance spectroscopy with counter-propagating beams, with a similar scheme to the excitation of the other fine transition 6 p 3/2 in the 6 p J manifold reported in Ponciano-Ojeda et al (2015 Phys. Rev. A 92 042511). The 5 p 3 / 2 → 6 p 1 / 2 dipole-forbidden excitation occurs at a wavelength λ ≈ 917.5 nm. The spectra showed the two main hyperfine states of the 6 p 1/2 manifold, and also resolved up to three satellites that appear due to effects of selection of velocities in thermal rubidium atoms. The polarization dependence of the relative intensities of the hyperfine lines of the decay fluorescence is also investigated. The data show that the individual line intensities and also the total cross-section have a strong dependence on the polarization direction. Experimental data for different polarization configurations of the light beams used in this two-photon spectroscopy are compared with the results of calculations that consider a strong atom-field coupling in the preparation step, followed by a weak electric quadrupole excitation and the blue fluorescence decay emission. Good agreement between experiment and this three-step model is found in the case of linear–linear polarizations, which confirms the results found for the 5 p 3 / 2 → 6 p 3 / 2 forbidden transition. The results presented here complete the information about the 5 p 3/2 → 6 p J electric dipole-forbidden transition in rubidium.
We report the laser cooling and confinement of Cd atoms in a magneto-optical trap and characterize the loading process from the background Cd vapor. The trapping laser drives the $^{1}S_{0}\ensuremath{\rightarrow}^{1}P_{1}$ transition at 229nm229\phantom{\rule{0.3em}{0ex}}\mathrm{nm}229nm in this two-electron atom and also photoionizes atoms directly from the 1P1^{1}P_{1}1P1​ state. This photoionization overwhelms the other loss mechanisms and allows a direct measurement of the photoionization cross section, which we measure to be $2(1)\ifmmode\times\else\texttimes\fi{}{10}^{\ensuremath{-}16}\phantom{\rule{0.3em}{0ex}}{\mathrm{cm}}^{2}$ from the 1P1^{1}P_{1}1P1​ state. When combined with nearby laser-cooled and trapped Cd+{\mathrm{Cd}}^{+}Cd+ ions, this apparatus could facilitate studies in ultracold interactions between atoms and ions.